Notes-Class-11-Science-Biology-Chapter-7-Cell Divisions-Maharashtra Board

Cell cycle :

• Sequential events occurring in the life of a cell is called cell cycle.
• Interphase and M - phase are the two phases of cell cycle.
• Cell undergoes growth or rest during interphase and divides during M — phase

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The interphase is subdivided into three sub-phases as G₁-phase, S-phase and G₂-phase.

G₁- phase (First gap period/First Gap Phase):

• It begins immediately after cell division.
• RNA (mRNA, rRNA and tRNA) synthesis, protein synthesis and synthesis of membranes take place during this phase.

S - phase (Synthesis phase):

• In this phase DNA is synthesized (replicated), so that amount of DNA per cell doubles.

• Synthesis of histone proteins takes place in this phase.
• If the initial amount of DNA is denoted as 2C then it increases to 4C. However, there is no increase in the chromosome number; if the cell had diploid or 2n number of chromosomes at G₁, even after S phase the number of chromosomes remains the same, i.e., 2n.
• DNA replication begins in the nucleus, and the centriole duplicates in the cytoplasm,

G₂- phase (Second growth phase/Second Gap Phase):

• Metabolic activities essential for cell division occur during this phase.
• Various proteins which are necessary for the cell division are also synthesized in this phase.
• Apart from this, RNA synthesis also occur during this phase.
• In animal cells, a daughter pair of centrioles appear near the pre-existing pair.

During all three phases of interphase, a cell grows by producing proteins and cytoplasmic organelles such as mitochondria and endoplasmic reticulum.

G₀ phase :

• G₀ is the phase of the cell cycle in eukaryotes in which many cell types stop dividing.
• If cells are deprived of appropriate growth factors, they stop at the G₁ checkpoint of the cell cycle.
• Their growth and division are arrested and they remain in G₀ phase.
• Mature neurons and muscle cells remain in G₀ phase.

G₀ phase is considered as either an extended G₁ phase in which the cell is neither dividing nor preparing to divide or a distinct quiescent stage which occurs outside the cell cycle. Cells in G₀ are in non-dividing phase.

There are three types of cell divisions:

(i) Amitosis :

• It is the simplest form of cell division. The nucleus elongates and a constriction appears along its length
• This constriction deepens and divides nucleus into two daughter nuclei followed by division of cytoplasm resulting in formation of two daughter cells.
• This type of division is observed in unicellular organisms, abnormal cells, old cells and in foetal membrane cells.

(ii) Mitosis:

• In this type of cell division, the cell divides and form two similar daughter cells which are identical to the parent cell.
• It is completed in two steps as karyokinesis and cytokinesis.

(iii) Meiosis:

• In this type of cell division, the number of chromosomes is reduced to half. Hence, this type of cell division is also called reductional division.
• Meiosis produces four haploid daughter cells from a diploid parent cell.

Prophase :

• In this phase, condensation of chromatin material, migration of centrosomes, appearance of mitotic apparatus and disappearance of nuclear membrane takes place.
• Due to condensation, each chromosome with its sister chromatids connected by centromere is clearly visible under light microscope.
• The nucleolus starts to disappear. ,
• Centrosome start moving towards the opposite poles of the cell.
• Mitotic apparatus is almost completely formed.

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Metaphase:

• Chromosomes are completely condensed and appear short.
• Centromere and sister chromatids become very prominent.
• All the chromosomes are arranged at equatorial plane of cell. This is called metaphase plate.
• Mitotic spindle is fully formed in this phase.
• Centromere of each chromosome divides into two, each being associated with a chromatid.

[Note: The centromeres divide at the beginning of anaphase so that the two chromatids of each chromosome become separated from each other.]

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Anaphase:

• In this phase, chromatids of each chromosome separate and form two chromosomes called daughter chromosomes.
• The chromosomes which are formed are pulled away in opposite direction by spindle apparatus.
• Anaphase ends when each set of chromosomes reach at opposite poles of the cell.

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Telophase :

• This is the final stage of karyokinesis.
• The chromosomes with their centromeres begin to uncoil at the poles.
• The chromosomes lengthen and lose their individuality.
• The nucleolus reappears and the nuclear membrane appear around the chromosomes. '
• Spindle fibres breakdown and get absorbed in the cytoplasm. Thus, two daughter nuclei are formed.

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Cytokinesis in animal cell :

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• Cytokinesis occurs by furrowing of plasma membrane that deepens and the daughter cells are formed.
• This step takes place at the end of karyokinesis (nuclear division) of mitosis.
• This process of division of cytoplasm is perpendicular to the spindle.
• It depicts the division of the cytoplasmic material in order to form two daughter cells that resemble each other.
• At the time of cytoplasmic division, organelles like mitochondria and plastids get distributed between the two daughter cells.
• Cleavage is present in this process.

Cytokinesis in plant cell :

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• Cytokinesis occurs by the formation and extension of cell plate from center to the periphery.

• Cleavage is absent in this process.
• The formation of the new cell wall begins with the formation of a simple precursor, called the ‘cell-plate’ that represents the middle lamella between the walls of two adjacent cells.

Significance of mitosis :

• As mitosis is equational division, the chromosome number is maintained constant.
• It ensures equal distribution of the nuclear and the cytoplasmic content between the daughter cells, both quantitatively and qualitatively. Therefore, the process of mitosis also maintains the nucleo-cytoplasmic ratio.
• The DNA is also equally distributed.
• It helps in growth and development of organisms.
• Old and worn-out cells are replaced through mitosis.
• It helps in the asexual reproduction of organisms and vegetative propagation in plants.
• Through mitotic divisions, meristematic tissues such as the apical and lateral cambium show continuous growth of plants throughout their life.

Life span of cell :

• Life span of different cells vary greatly.
• Life span of a cell is decided by its growth rate, metabolic activities and cell size.
• The life span of a cell can be analysed in laboratory by applying carbon-14 technique to DNA. This method is commonly used in archaeology and paleontology to find the age of fossils. Same can be applied to determine the life span of a cell.

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Leptotene :

• The volume of the nucleus increases.
• The chromosomes become long distinct and coiled.
• They orient themselves in a specific form known as bouquet stage. This is characterized with the ends of chromosomes converged towards the side of nucleus where the centrosome lies.
• The centriole divides into two and migrate to opposite poles,

Zygotene :

• Pairing of non-sister chromatids of homologous chromosomes takes place by formation of synaptonemal complex. This pairing is called synapsis.
• Each pair consist of a maternal chromosome and a paternal chromosome.
• Chromosomal pairs are called bivalents or tetrads.

Pachytene:

• Each individual chromosome begins to split longitudinally into two similar chromatids.
• Therefore, each bivalent now appears as a tetrad consisting of four chromatids.
• The homologous chromosomes begin to separate but they do not separate completely and remain attached to one or more points.
• These points are called chiasmata (Appear like a cross-X).
• Chromatids break at these points and broken segments are exchanged between non-sister chromatids of homologous chromosomes resulting in recombination.

Diplotene :

• The chiasmata become clearly visible in diplotene due to beginning of repulsion between synapsed homologous chromosomes. This is known as desynapsis.
• Synaptonemal complex also starts to disappear.

Diakinesis :

• The chiasmata beings to move along the length of chromosomes from the centromere towards the ends of chromosomes. The displacement of‘ chiasmata is termed as terminalization.
• The terminal chiasmata exist till the metaphase.
• The nucleolus and nuclear membrane completely disappear and spindle fibres begin to appear. .

Synaptonemal complexes are zipper like structures assembled between homologous chromosomes during the prophase of first meiotic division.

Spindle fibres :

• Spindle fibres are formed from microtubules with many accessory proteins.
• Spindle fibres elongate for assembly of chromosomes at equatorial plane of the cell during metaphase and spindle fibres contract for pulling chromosomes towards opposite poles during anaphase.
• The spindle fibres elongate (polymerize) by incorporating subunits of the protein tubulin and contract (depolymerize) by losing subunits.

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Prophase — II:

• The chromosomes are ‘distinct with two chromatids.
• Each centriole divides into two resulting in formation of two centrioles which migrate to opposite poles and form asters.
• Spindle fibres are formed between the centrioles.
• The nuclear membrane and nucleolus disappears in this phase.

Metaphase — II:

• Chromosomes are arranged at the equator.
• The two chromatids of each chromosome are separated by division of the centromere.
• Some of the spindle fibres are attached to the centromeres and some are arranged end to end between two opposite centrioles.

Anaphase — II:

In this phase, the separated chromatids become daughter chromosomes and move to opposite poles due to the contraction of the spindle fibres attached to centromeres.

Telophase — II:

• In this stage daughter chromosomes starts to uncoil.
• The nuclear membrane surrounds each group of chromosomes.
• Nucleolus reappears in this phase.

Cytokinesis — II

• Cytokinesis takes place after the nuclear division.
• Two haploid cells are fonned from each haploid cell.
• Thus, four haploid daughter cells are formed.
• These cells then undergo changes to form gametes.

Difference between meiosis — I and meiosis — II :

Process of recombination :

• Recombination is exchange of genetic material between paternal and maternal chromosomes during gamete formation.
• The points where crossing over takes place is known as chiasmata.
• Chromatids acquire new combinations of alleles by physically exchanging segments in crossing-over.
• The exchange of genetic material between homologous chromosomes involves accurate breakage an joining of DNA molecules through a complex mechanism.
• It is catalyzed by enzymes.